Battery monitoring system using time-based signals

ABSTRACT

Systems and methods for monitoring a battery stack having a plurality of cells are provided. One system includes a plurality of time delay circuits. Each of the time delay circuits is electrically coupled to at least one of the plurality of cells. The time delay circuits are configured to execute a time delay in response to receiving a trigger signal and output a time delay marking signal indicating that the time delay has elapsed. The time delay is based on a voltage of the cell(s) to which the respective time delay circuit is electrically coupled. The system further includes a control circuit configured to receive the time delay marking signal from each of the time delay circuits and, for each received time delay marking signal, to determine the voltage of the at least one of the plurality of cells associated with the respective time delay marking signal.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application claims priority to and the benefit of U.S.Provisional Patent Application No. 61/621,714, filed Apr. 9, 2012, theentire disclosure of which is incorporated herein by reference.

BACKGROUND

The present disclosure relates generally to the field of monitoringbattery cells. More specifically, the present disclosure relates tosystems and methods for monitoring battery cells using time-basedsignals.

Battery stacks or systems (e.g., battery modules, battery packs, etc.)that include multiple battery cells are used in a variety ofapplications. For example, battery stacks may be used in suchapplications as electric and/or hybrid vehicles, uninterruptible powersupply systems, power tools, and other types of applications. Chargingand discharging battery cells to a level outside of an acceptableoperating range can result in damage to the cells and decrease the lifecycle of the cells. By monitoring the voltages of the battery cells, thebattery cells can be controlled to ensure that they are maintainedwithin an acceptable operating range that will extend the life cycle ofthe cells. This is particularly important with certain batterychemistries, such as lithium chemistries (e.g., lithium-ion,lithium-polymer, etc.) that are popular with various applicationsincluding those mentioned above.

SUMMARY

One embodiment of the disclosure relates to a system for monitoring abattery stack having a plurality of cells that includes a plurality oftime delay circuits. Each of the time delay circuits is electricallycoupled to at least one of the plurality of cells. The time delaycircuits are configured to receive a trigger signal, execute a timedelay in response to receiving the trigger signal, and output a timedelay marking signal indicating that the time delay has elapsed. Thetime delay is based on a voltage of the at least one of the plurality ofcells to which the respective time delay circuit is electricallycoupled. The system further includes a control circuit configured toreceive the time delay marking signal from each of the time delaycircuits and, for each received time delay marking signal, to determinethe voltage of the at least one of the plurality of cells associatedwith the respective time delay marking signal.

Another embodiment relates to a method for monitoring a battery stackhaving a plurality of cells. The method includes, for each of aplurality of subsets of the plurality of cells, executing a time delaythat is proportional to a voltage of the subset of cells. Each subset ofthe plurality of cells includes one or more cells. The method furtherincludes, for each of the plurality of subsets of the plurality ofcells, activating a time delay marking signal when the time delay haselapsed. The method further includes, for each time delay markingsignal, determining the voltage of the associated subset of cells basedon the time delay marking signal.

Another embodiment relates to a system for monitoring a battery stackhaving a plurality of cells. The system includes means for executing,for each of a plurality of subsets of the plurality of cells, a timedelay that is proportional to a voltage of the subset of cells. Eachsubset of the plurality of cells includes one or more cells. The systemfurther includes means for activating, for each of the plurality ofsubsets of the plurality of cells, a time delay marking signal when thetime delay has elapsed. The system further includes means fordetermining, for each time delay marking signal, the voltage of theassociated subset of cells based on the time delay marking signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a battery monitoring system according to anexemplary embodiment.

FIG. 2 is an illustration of time delay circuits and marking signalsthat may be generated by the time delay circuits according to anexemplary embodiment.

FIG. 3 is a schematic diagram of a battery monitoring circuit includingtime delay circuits configured to monitor three battery cells accordingto an exemplary embodiment.

FIG. 4 is an illustration of marking signals that may be generated usingthe battery monitoring circuit of FIG. 3 according to an exemplaryembodiment.

FIG. 5 is a schematic diagram of another battery monitoring circuitaccording to an exemplary embodiment.

FIGS. 6A and 6B are graphs illustrating signals that may be generatedusing the battery monitoring circuit of FIG. 5 according to an exemplaryembodiment.

FIG. 7 is a flow diagram of a process for monitoring cells of a batterystack according to an exemplary embodiment.

FIG. 8 is a block diagram of a battery monitoring system according toanother exemplary embodiment.

FIG. 9 is a graph illustrating signals that may be generated using thebattery monitoring system of FIG. 8 according to an exemplaryembodiment.

FIG. 10 is a circuit diagram of an analog portion of a delay module thatmay be used in the implementation of the battery monitoring system ofFIG. 8 according to an exemplary embodiment.

FIG. 11 is a circuit diagram of a logic portion of a delay module thatmay be used in the implementation of the battery monitoring system ofFIG. 8 according to an exemplary embodiment.

DETAILED DESCRIPTION

Before turning to the figures, which illustrate the exemplaryembodiments in detail, it should be understood that the application isnot limited to the details or methodology set forth in the descriptionor illustrated in the figures. It should also be understood that theterminology is for the purpose of description only and should not beregarded as limiting.

Referring generally to the figures, systems and methods are providedthat may be used to monitor the voltage of battery cells. Batterymonitoring systems are important for many applications to preventshortening of battery life. For example, lithium battery cells must notbe overcharged or over-discharged or the cells may be damaged and cyclelife may be reduced. In some applications, the cost of batterymonitoring and management circuitry can be a significant portion of thetotal costs for the battery stack.

Measuring cell voltages in battery stacks may involve a variety ofchallenges. Voltage measurement of individual cells may require a highdegree of accuracy (e.g., better than 1 percent) for some applications.Accuracy may be particularly important at high and low cell chargelevels. For example, lithium cells have a flat discharge curve (e.g.,may change only 5 mV per 10 percent State of Charge (SOC) change in themidrange of the curve) with a steep voltage increase at a high end ofthe charge range and a steep voltage decrease at the low end of thecharge range, requiring particular accuracy at the charging extremes.Additionally, relatively small cell voltages (e.g., 2.5V to 4V) may bemeasured and communicated in the presence of very large common modestack voltages (e.g., 300V to 400V across the battery stack). Parasiticcurrent draw from measurement circuits can also create an imbalance inthe cells of the battery stack over time. Ideally, a battery monitoringcircuit will induce no or very little (e.g., in the range of microamps)parasitic current draw. Hot plugging of cells or high current terminalconnections within the battery stack (e.g., connecting batterymeasurement systems to the cells during operation) can create abnormallylarge reversal potentials when measured by the measurement circuit.These potentials can damage measurement devices if such devices are notadequately protected. Some applications, such as electric vehicles,involve motor inverter chopping frequencies (e.g., 10 kHz to 20 kHz)that should be accounted for by the battery monitoring system. Pulsewidth modulated chopping drives for high power applications likeelectric vehicles may turn on and off the current flowing through thebattery stack very rapidly. Such cycling can create a very unstable cellvoltage environment.

Various solutions may be used to monitor the cells of a battery stackand determine the voltages of the cells. One solution may utilize aflying capacitor that is charged up to the cell voltage and thenswitched to a chassis-referenced potential. This solution requires highvoltage switches to measure the cell voltages. A clamped transformersolution includes a small transformer for each cell with the secondarywinding clamped by the cell. This solution utilizes high cost componentsand is susceptible to vibration.

Other solutions may utilize integrated circuits configured to monitorcells of a battery stack. A single cell analog-to-digital converter(ADC) solution may use locally referenced ADC circuitry and transmit abinary code representing cell voltages across a potential gradient ofthe battery stack using a daisy chained level shifting serial portinterface. A grouped cell ADC solution may use shared ADC circuitry aswell as circuitry configured to reference, range, and multiplex theanalog cell voltages to match a shared ADC input circuit. Thesesolutions require localized clocked timing and digital processing ofanalog signals. The grouped cell ADC solution may also be limited by theamount of common mode voltage it can accommodate, limiting the cellgrouping size (e.g., no more than 12 cells). To overcome the voltagelimitation, the grouped ADC solution may include a daisy chained serialport. The single cell and shared ADC solutions may be higher cost thanthe battery monitoring systems and methods discussed below and mayinduce higher parasitic current draw on the battery cells.

The exemplary embodiments discussed herein provide systems and methodsthat may be used to monitor battery cell information such as cellvoltages based on time-based signals. Time delay circuits may beelectrically coupled to cells of a battery stack and used to convertvoltages of the cells into a time-based representation in which thevoltages are represented by a time interval (e.g., a time intervalbetween output pulses or marking signals). The time delay circuits mayoutput time-based marking signals or pulses that may be received by acontrol circuit. The control circuit may decode the time-based markingsignals by using time intervals associated with the marking signals(e.g., time intervals between marking signals and trigger signals and/ortime intervals between adjacent marking signals) to determine thevoltages of the respective battery cells. In some embodiments, timedelay circuits may be daisy chained to one another and a marking signalof one time delay circuit may be used to trigger the time delayoperation of the next time delay circuit, such that the time delays foradjacent cells occur in a continuous and consecutive manner.

One or more of the exemplary embodiments presented herein may providevarious advantages. The problem of measuring and communicating smallvoltages in the presence of high common mode voltages may be overcome byperforming locally referenced voltage to time conversions and usingpulse position modulation to encode and transmit voltage in a time-basedrepresentation as marking signals. In some embodiments, the timeinterval information may be inherently preserved because the triggersignals triggering the beginning of the time delay operation and themarking signals marking the end of the delay may be transmitted throughthe same digital propagation delay path. In some embodiments, the samemarking signal used to transmit voltage information in a time-basedrepresentation to a control circuit may be used to trigger the nextvoltage-to-time conversion sample, creating a sequential sampling chainreaction and avoiding the need for the control circuit or anothercircuit to send separate trigger signals to each time delay circuit.Some embodiments may use simpler circuitry, have lower cost per cell,and/or have lower quiescent current draw and parasitic load onindividual battery cells as compared to other battery monitoringsolutions. Some embodiments may allow monitoring of battery stackswithout a limit, or with a very high limit, in the number of cells thatmay be monitored per control circuit (e.g., per decoding processor). Theuse of time-based representation may maintain a continuousrepresentation of the analog voltage of cells, which may allow for ahigher resolution if later converted into digital signals. Someembodiments may utilize a single point of high voltage galvanicisolation for signaling, reducing cost as compared to performinggalvanic isolation at multiple input points and using multiple separatedevices. Some embodiments may apply relatively long integration timesfor cell voltage sampling to average out sampling noise (e.g., frompulse width modulated chopping drives of electric vehicles), which mayallow for higher data resolution and more stable measurements than othermeasurement devices.

Referring now to FIG. 1, a block diagram of a battery monitoring system100 is provided according to an exemplary embodiment. System 100 may beused to monitor information relating to a battery stack having multiplebattery cells. The battery cells may be connected to one another in aseries configuration. In some embodiments, the battery cells may insteadbe connected in a different configuration, such as a parallel or aseries-parallel configuration. System 100 may be used to monitor thevoltages of individual battery cells or subsets of cells based ontime-based signals. System 100 may use the determined voltages tomonitor and/or record voltages for each of the cells, provide batterycell information to other devices and/or to users, monitor the health ofthe battery cells and determine if one or more cells have been damaged,balance the charge of a group of cells, and/or perform other actionswith respect to the battery cells. System 100 may be used with any typeof rechargeable (i.e., secondary) battery, such as NiCd, lead-acid,NiMH, metal-air batteries (e.g., Zn-air, Li-air, etc.), lithium-ion,lithium-polymer, etc.

System 100 includes multiple time delay circuits 132-138 electricallycoupled to cells 122-128 of the battery stack and configured to convertthe voltages of cells 122-128 into time-based marking signals andtransmit the marking signals to a control circuit 105. The time delaycircuits 132-138 are configured to receive a trigger signal (e.g., fromcontrol circuit 105) and to initiate a time delay operation. The lengthof the time delay is based on (e.g., proportional to) the voltage (and,accordingly, the charge level) of the respective cell or cells to whichthe time delay circuit is coupled. Once the time delay has elapsed, thetime delay circuits 132-138 activate a marking signal indicating thatthe time delay has elapsed. The marking signal may be a brief pulse thatis activated (e.g., changed from a low to high level) once the timedelay has elapsed and shortly thereafter deactivated (e.g., changed backto the low level). The time-based marking signal may be transmitted tothe control circuit 105, which may be configured to decode the markingsignal and determine the voltage of the cell associated with the markingsignal. In the illustrated exemplary embodiment, each time delay circuitis coupled to a single cell of the battery stack. In other embodiments,each time delay circuit may be coupled to a subset of the cells of thebattery stack, such that one or more of the time delay circuits may becoupled to multiple cells and generate a time-based representation ofthe voltage of the multiple cells. System 100 may be scaled to monitor abattery stack having any number of cells (e.g., 100 or more cells) byadding additional time delay circuits to monitor the cells.

In some embodiments, one or more of the time delay circuits 132-138 maybe configured to generate and transmit a calibration marking signalrepresentative of a time delay associated with reaching a fixedfractional voltage. The calibration marking signal may be used tocalibrate the control circuit 105 and may allow control circuit 105 todecode the marking signals and obtain the voltage information withoututilizing the specific component values (e.g., resistor and capacitorvalues) used in time delay circuits 132-138. In some embodiments,control circuit 105 may utilize the particular component values incalculating the voltages rather than receiving a calibration markingsignal.

In some embodiments, time delay circuit 132-138 may be connected ordaisy chained to one another and configured to trigger one another in asequential manner. For example, a first trigger signal may betransmitted to time delay circuit 132 (e.g., by control circuit 105),causing time delay circuit 132 to initiate a time delay operation basedon the voltage of cell 122. Once the time delay of time delay circuit132 has elapsed, time delay circuit 132 may output a time-based markingsignal that is transmitted to control circuit 105. The marking signaloutput by time delay circuit 132 may also be transmitted to time delaycircuit 134 and used as the trigger signal to cause time delay circuit134 to begin the next time delay operation for cell 124. This processmay continue and marking signals may be transmitted from one time delaycircuit to the next in a chain causing the time delay operations of timedelay circuits 132-138 to execute sequentially. Control circuit 105 maybe configured to receive the marking signals from each time delaycircuit and to determine the voltages for each cell based in part on thetime delay between adjacent marking signals. In some embodiments, timedelay circuits 132-138 may not be chained to one another, and separatetrigger signals may be transmitted from control circuit 105 to timedelay circuits 132-138 to cause time delay circuits 132-138 to executetime delay operations.

Control circuit 105 is configured to receive marking signals from timedelay circuits 132-138 and to determine voltages associated with cells122-128 based on the marking signals. Control circuit 105 may include atleast one processor 110 configured to execute functions of controlcircuit 105 and at least one memory 115 configured to store data andinstructions to be executed by processor 110. Processor 110 may be anytype of general or special purpose processor (e.g., FPGA, CPLD, ASIC,etc.), and memory 115 may be any type of machine readable medium (RAM,ROM, flash memory, etc.).

Control circuit 105 may include an input interface 140 configured toreceive the time-based marking signals from time delay circuits 132-138.In some embodiments, multiple or all of the marking signals may bereceived through a single input of input interface 140. The single inputmay indicate the presence of a marking signal when any of the markingsignals output by time delay circuits 132-138 are activated. When timedelay circuits 132-138 are chained to one another and used to triggerone another, control circuit 105 can determine which of cells 122-128 amarking signal is associated with based on how many marking signals havepreviously been received. For example, the third marking signal receivedby control circuit 105 through the single input may be determined to beassociated with the third cell 126. In some embodiments, separate inputsof input interface 140 may be used for separate marking signals.

In some embodiments, control circuit 105 may be isolated from the highvoltage battery stack using a galvanic isolation circuit 180. Signalsfrom time delay circuits 132-138 may be received by galvanic isolationcircuit 180, and galvanic isolation circuit 180 may be configured totransmit the signals forward to control circuit 105 without permittingdirect current flow between time delay circuits 132-138 and controlcircuit 105. Control circuit 105 may operate at an independent voltageand reference/ground voltage than the high voltage at which the batterypack operates through use of galvanic isolation circuit 180. Galvanicisolation circuit 180 may include an object such as an opto-isolatorcircuit (e.g., a circuit configured to transmit signals as opticalsignals) to transmit cell data from time delay circuits 132-138 tocontrol circuit 105 across the high voltage boundary between thedevices. Some embodiments may permit data transfer with significantlyreduced costs by using a single point of monitoring. For example, alloutputs of time delay circuits 132-138 may be routed through a singletransmission path to a single input of control circuit 105. This mayrequire only a single galvanic isolation circuit 180 (e.g., a singleopto-isolator circuit) for all of time delay circuits 132-138 as opposedto a separate galvanic isolation circuits for each time delay circuit.Transmitting data between time delay circuits 132-138 may be lesscomplex as opposed to sending it directly to control circuit 105 becausethe voltage differential between cells 122-128 is typically much lowerthan the voltage difference between each of cells 122-128 and controlcircuit 105.

Memory 115 may include a decoding module 150 configured to decode thetime-based marking signals received from time delay circuits 132-138 andto recover the voltages of cells 122-128. In some embodiments, decodingmodule 150 may determine voltages based upon the time between markingsignals and knowledge of component values of time delay circuits132-138. In some embodiments, decoding module 150 may determine voltagesbased upon the time between marking signals and a configuration markingsignal representative of the time to reach a fixed fractional voltage.Use of a fixed fractional reference voltage allows decoding module 150to determine voltages without utilizing the specific component (e.g.,resistor and capacitor) values of time delay circuits 132-138. In someembodiments, control circuit 105 may be configured to store theresultant voltage values for cells 122-128 in a database 160. Voltagevalues for cells 122-128 may be tracked over time and may be used todetermine the health of cells 122-128. In some embodiments, voltagevalues may be transmitted through an output interface 145 to a computingdevice 165 configured to perform additional calculations or functionsusing the voltage values and/or to a display 170 configured to displaythe voltage data to a user.

In some embodiments, memory 115 may include a control module 155configured to generate control signals used to control the charging anddischarging of cells 122-128. For example, control module 155 may beconfigured to generate control signals configured to cause the voltagelevels of cells 122-128 to become balanced when it is determined thatthere is a substantial charge imbalance between cells. Control module155 may be configured to charge or discharge cells when cells aredetermined to have a voltage outside of a normal or safe operating rangefor the cells. In some embodiments, control module 155 may send signalsto a separate charging module or device configured to charge anddischarge the battery stack. In some embodiments, one or more modules(e.g., decoding module 150 and/or control module 155) shown as beingimplemented as instructions stored within memory 115 may be additionallyor alternatively implemented using one or more hardware circuits.

In some embodiments, one or both of time delay circuits 132-138 andcontrol circuit 105 may be embedded within and/or sold as a unit withthe battery stack. In some embodiments, time delay circuits 132-138and/or control circuit 105 may generate and/or decode signals withoutusing a multiplexer, level shifter, analog to digital converter, orserial port. In some embodiments, marking signal generation and decodingmay be performed without using a reference voltage, such as by utilizinga voltage of a previously analyzed cell as a comparison voltage.

Referring now to FIG. 2, an illustration of time delay circuits 205 andmarking signals 255 that may be generated by time delay circuits 205 isshown according to an exemplary embodiment. A beginning trigger signal235 may be used to trigger the time delay operation of a first timedelay circuit. Once the first time delay operation has completed, amarking signal may be generated and sent along a transmission path 225to a monitoring point 230 (e.g., an input of a control circuit) wherethe marking signal may be received and decoded to determine a voltage ofa cell being monitored using the first time delay circuit. The markingsignal may also be sent along a transmission path 220 to a second timedelay circuit and used as a trigger signal for the second time delaycircuit to begin a second time delay operation for another cell or groupof cells. This cascading sequence of pulse marked time delays maycontinue, with each marking signal being transmitted toward monitoringpoint 230 and also used to trigger the next time delay circuit, until alast time delay circuit has completed a time delay operation and thelast marking signal has been transmitted toward monitoring point 230.All of the marking signals may be transmitted along the sametransmission path and electrically combined (e.g., using an electrical“OR”) in a manner such that activation of a marking signal by any oftime delay circuits 205 will register as an active marking signal atmonitoring point 230. A processing circuit connected at monitoring point230 may determine the voltages of the cells associated with time delaycircuits 205 based in part on the amount of time between sequentialmarking signals 255.

Referring now to FIG. 3, a schematic diagram of a battery monitoringcircuit 300 including time delay circuits configured to monitor threebattery cells is shown according to an exemplary embodiment. In someembodiments, multiple of the circuits shown in FIG. 3 may be chainedtogether to monitor battery stacks having more than three cells. Circuit300 utilizes an exponential ramp and compare operation for encoding anddecoding of voltage information. In other embodiments, circuit 300 maybe modified to utilize a linear ramp and compare operation. Circuit 300is able to achieve voltage conversion of three cells using lesscomponents than if separate time delay circuits were used for each cellbecause the voltage reference and ramp circuit is shared between thethree cells. The accuracy of the ratio between cells is also not limitedby the accuracy of the voltage reference.

Circuit 300 includes a quad comparator, a voltage reference, a voltagedivider, and a capacitor charge ramp. Four pulses are generated bycircuit 300 to represent the three cell voltages. A cell couplingconnector 308 may be used to couple circuit 300 to the battery cells,with one cell connected across terminals 1 and 2 of connector 308,another cell connected across terminals 2 and 3, and the third cellconnected across terminals 3 and 4. A connector 302 may receive an inputenable or trigger signal, labeled EN_IN, configured to begin a timedelay operation of circuit 300. When the trigger signal is received, atransistor 310 is enabled and a capacitor C1 begins charging. Oncecapacitor C1 has charged to a reference voltage Vr (e.g., 1.182V), acomparator 312 activates output OUTA, which in turn activates atransistor 322 and outputs a first time-based marking signal through aDATA_OUT output of connector 302. Once a capacitor C1 is charged to avoltage V1 that is the equal to the voltage of the first cell, acomparator 314 activates output OUTB, which in turn activates transistor324 and outputs a second marking signal through the DATA_OUT output ofconnector 302. Once capacitor C1 is charged to a fractional voltagedefined by a voltage divider consisting of resistors R3 and R4, acomparator 316 activates output OUTC, which in turn activates transistor326 and outputs a third marking signal through the DATA_OUT output ofconnector 302. Once capacitor C1 is charged to a voltage V2 equal to thecombined voltage of the first and second cells, a comparator 318activates output OUTD, which in turn activates transistor 328 andoutputs a fourth marking signal through the DATA_OUT output of connector302.

Additional cells may be monitored by connecting multiple circuits in achain configuration by connecting a connector 302 of one circuit to aconnector 304 of another circuit. The OUTD signal activated bycomparator 318 may also be used to activate a transistor 330 and toenable an EN_OUT output of connector 304. This output may be connectedto an EN_IN input of an adjacent time delay circuit and may beconfigured to begin the time delay operation of that circuit. Markingsignals output by the adjacent circuit may be transmitted through theDATA_OUT output of that circuit to the DATA_IN input of connector 304and propagated through transistor 320 to the DATA_OUT output ofconnector 302. In this manner, marking signals of multiple time delaycircuits may be transmitted through a single transmission path to asingle input of a control or measurement circuit.

A control circuit may be configured to receive the marking signalsoutput by circuit 300 through the DATA_OUT output of connector 302 anddetermine the voltages of the battery cells based on the markingsignals. The following values may be used by the control circuit indetermining the voltages:

-   -   Vr=Voltage Reference (e.g., 1.182V)

f=R4/(R3+R4)=0.5

-   -   Tr=time to charge C1 to reference voltage Vr and activate OUTA        (i.e., time from activation of enable signal EN_IN to first        marking signal)    -   T1=Time to Charge C1 to V1 and Activate OUTB (i.e., time from        activation of enable signal EN_IN to second marking signal)    -   Tf=Time to Charge C1 to Fractional Voltage and Activate OUTC        (i.e., time from activation of enable signal EN_IN to third        marking signal)    -   T2=Time to Charge C1 to V2 and Activate OUTD (i.e., time from        activation of enable signal EN_IN to fourth marking signal)

The following exponential values may be determined by the controlcircuit based on the determined times between received signals:

er=exp(Tr/Tf*ln(1−f))

e1=exp(T1/Tf*ln(1−f))

e2=exp(T2/Tf*ln(1−f))

The control circuit may use these values to determine the voltages ofeach of the cells as follows:

Cell 1=(1−e1)*Vr/(1−er)

Cell 2=(e1−e2)*Vr/(1−er)

Cell 3=e2*Vr/(1−er)

In the equations above, the expression Vr/(1−er) is equivalent to thecombined three cell voltage V3 of the cells monitored by circuit 300. Itis also apparent that Vr/(1−er) will cancel out if calculating the ratioof cell voltages.

Referring now to FIG. 4, an illustration 400 of marking signals that maybe generated using circuit 300 is shown according to an exemplaryembodiment. A capacitor (e.g., capacitor C1 of circuit 300) graduallycharges to a combined voltage level of three cells 405, 410, and 415connected in series. A voltage divider consisting of two resistors 420and 425 is used to generate a fractional voltage used for timebasecorrection and configuration. The RC timebase of circuit 300 can beinfluenced by temperature and component values, and the extra referencepulse may be used to compensate for this drift. The first marking signal435 is generated when the capacitor charges to a fixed reference voltage(e.g., when OUTA in circuit 300 is activated by comparator 312). Thesecond marking signal 440 is generated when the capacitor charges to thevoltage of first cell 405 (e.g., when OUTB in circuit 300 is activatedby comparator 314). The third marking signal 445 is generated when thecapacitor charges to the fractional voltage defined by the voltagedivider (e.g., when OUTC in circuit 300 is activated by comparator 316).The fourth marking signal 450 is generated when the capacitor charges tothe voltage equal to the combined voltage of first cell 405 and secondcell 410 (e.g., when OUTD in circuit 300 is activated by comparator318). Fourth marking signal 450 may be used as the trigger signal foranother time delay circuit.

Referring now to FIG. 5, a schematic diagram of another batterymonitoring circuit 500 is shown according to an exemplary embodiment.Circuit 500 requires only a single wire bus (e.g., North/South bus) fordata, trigger, and voltage reference signals. Circuit 500 also does notrequire a separate voltage reference signal because the voltage of anadjacent cell is used as a reference. In some embodiments, circuit 500may be used within a custom 4-pin integrated circuit for a solution tobe embedded within the battery stack.

Circuit 500 uses a single wire bus by which signals are communicated toand from an upstream adjacent circuit (a circuit before circuit 500 in achain of circuits) through a line labeled data3 and signals arecommunicated to and from a downstream adjacent circuit (a circuit aftercircuit 500 in the chain) through a line labeled data2. When a signal ondata3 is released after being substantially high (e.g., indicating atrigger or marking signal of the previous time delay circuit),transistor Q18 is momentarily activated by transistors Q12 and Q17. Theactivation of transistor Q18 in turn activates transistor M2, which islatched into an “ON” state by comparator U3. Data2 is latched into ahigh state when data3 is released. Capacitor C5, which is normallycharged to the voltage across the neighboring cell to the south, ispulled to the positive rail of the active cell by transistor M2 andbegins discharging after data3 is released. A voltage of Vcell plus thevoltage across the neighboring cell to the south, Vcell(south), is seenat the negative inputs of comparators U3 and U4. Comparator U3 continuesto latch transistor M2 into the “ON” state until the voltage acrosscapacitor C5 drops to half of the voltage Vcell of the cell beingmonitored by circuit 500, based on the voltage divider consisting ofresistors R18 and R27. Once the voltage across capacitor C5 dischargesto Vcell, a first marking signal pulse is generated by comparator U4 andtransistor Q9 and is transmitted across data3 by transistor Q11. Oncethe voltage across capacitor C5 discharges to 0.5*Vcell, another markingsignal pulse is generated by comparator U4 and transistor Q8 andtransmitted across data3 by transistor Q11. Data2 is released into a lowstate, triggering the time delay operation of the next time delaycircuit. Transistor Q10 monitors data2 for data signals traveling backthrough data2 to be transmitted across data3. Transistors Q8 and Q9monitor for marking signals generated by comparators U3 and U4 ofcircuit 500. When transistors Q8, Q9, and/or Q10 are activated,transistor Q11 is activated and the signals are propagated north acrossdata3. The data signals may be transmitted to and monitored at thebeginning of the data bus. A control circuit connected to the beginningof the data bus may determine the relative voltage of adjacent cellsaccording to the following equation:

Vcell(n−1)Ncell(n)=exp(ln(2)*time1/time2)−1

The terms time1 and time2 in the equation above refer to the delay timeassociated with the first and second marking signal pulses generated bycomparators U4 and U3, respectively. Because circuit 500 uses dualcomparison events, the RC time constant is cancelled out. Incorporatingthe voltage of the neighboring cell Vcell(south) into the discharge rampallows time intervals to be produced that represent a ratio ofneighboring cell voltages. The second marking signal and associated timeinterval can be used to cancel out RC component values from the voltagedecoding.

Referring now to FIG. 6A, a graph 600 illustrating signals that may begenerated by multiple circuits like circuit 500 is shown according to anexemplary embodiment. A signal 605 may appear on an uppermost portion ofthe signal bus labeled data4. A signal 610 is the signal appearing onthe upper portion of the signal bus in circuit 500. A signal 615 is thesignal appearing on the lower portion of the signal bus in circuit 500.

When signal 610 goes into a low state after being high at approximately25 ms, the time delay operation of circuit 500 begins. Two markingsignal pulses 630 occur when the voltage across capacitor C5 dischargesto Vcell and 0.5*Vcell. After the voltage across capacitor C5 reaches0.5*Vcell and the second pulse occurs on data3, signal 615 on data2 goeslow, starting the delay operation of the next time delay circuit. Whenthe marking signal pulses occur in the next time delay circuit, theypropagate up through data3 and data4. The control circuit may monitorthe data pulses and perform conversions using signal 605 on data4, whichwill include all of the marking signal pulses of subsequent circuits. Insome embodiments, an incoming data pulse (e.g., as reflected in signal620) during an active state time slot for a particular module could beused to expand functionality of the circuit, such as to latch shunt aresistor for dissipative charge balancing.

Referring to FIG. 6B, a second graph 650 illustrating additional signalsthat may be generated by multiple circuits like circuit 500 is shownaccording to an exemplary embodiment. Graph 650 illustrates the signals610 and 615 representing the signals that may be seen at data3 anddata2, respectively, in circuit 500 during operation. Graph 650 alsoillustrates a signal 660 that represents a voltage that may be seen atthe negative input of comparators U3 and/or U4 during operation. Whendata3 signal 610 goes high, the voltage across capacitor C5 seen at theinput of comparators U3 and U4 jumps to approximately Vcell (the voltageof the cell monitored by circuit 500)+Vcell(south) (the voltage of theneighboring cell to the south). When data3 signal 610 goes low,capacitor C5 begins to discharge in the manner illustrated by signal660. When capacitor C5 discharges to a voltage equal to Vcell (see point665), the first marking signal is triggered at a time labeled timelafter the falling edge of data3 signal 610. When capacitor C5 dischargesto a voltage equal to 0.5*Vcell (see point 670), the second markingsignal is triggered at a time labeled time2 after the first markingsignal was triggered. Time1 and time2 may be used to determine therelative voltages of adjacent cells in accordance with the equation andprocess described above.

Referring now to FIG. 7, a flow diagram of a process 700 that may beused to monitor cells of a battery pack is shown according to anexemplary embodiment. In some embodiments, process 700 may be carriedout using one or more components of system 100, circuit 300, and/orcircuit 500.

A battery monitoring system may be configured to execute a time delayusing a time delay circuit associated with one or more cells of abattery pack (705). The time delay is based on (e.g., proportional with)the voltage of the one or more cells. Once the time delay has elapsed, amarking signal is activated to indicate the end of the time delay (710).The voltage of the cell(s) may be determined based on the timingassociated with the marking signals (715). If there are further cells tobe monitored in the battery stack, a trigger signal may be sent to thenext time delay circuit to begin the next time delay. In someembodiments, the time delay circuits may be chained together and themarking signals of one circuit may be used to trigger the beginning ofthe time delay operation for the next circuit. Once time delays havebeen executed and marking signals have been generated for all cells,process 700 ends (725).

Referring now to FIG. 8, a block diagram of another battery monitoringsystem 800 is shown according to an exemplary embodiment. System 800includes five delay modules 855, 860, 865, 870, and 875 configured tomeasure and represent information (e.g., voltages) about battery cells805, 810, 815, 820, and 825, respectively, as time-based signals. System800 may be an interleaved dual slope delay module system in whichtime-based signals representing voltages for each of cells 805, 810,815, 820, and 825 are generated by performing integrating anddeintegrating operations in each of delay modules 855, 860, 865, 870,and 875. The integrating operation of one delay module may be performedduring a same time period as the deintegrating operation of an adjacentdelay module. A trigger signal may be transmitted from a controller 835to first delay module 855 through a galvanic isolation module 830 andmay trigger the first integration and deintegration delay cycle of firstdelay module 855. Once the integration operation of first delay module855 has concluded, the integration operation of second delay module 860may be triggered and may continue during the deintegration operation offirst delay module 855. This process may continue in a chained manneruntil the integration and deintegration cycles for all delay modules855-875 have completed. Marking signals may be triggered at the end ofeach deintegration cycle that may be used by controller 835 to determinevoltages for each of cells 805-825. Delay modules 855-875 are connectedto controller 835 through a single wire daisy chain configuration,allowing for a single monitoring point and single point of galvanicisolation. System 800 may be scaled to monitor any number of cellsaccording to various embodiments.

Referring now to FIG. 9, a graph 900 illustrating signals that may begenerated using battery monitoring system 800 is shown according to anexemplary embodiment. A horizontal time axis 905 of graph 900 representselapsed time, and a vertical voltage axis 910 represents voltagesassociated with the signals. Graph 900 includes integrator outputsignals 920, 925, 930, 935, and 940 associated with delay modules 855,860, 865, 870, and 875, respectively, and illustrating the integrationand deintegration operations of the delay modules over time. Graph 900also includes a first module south signal 915 illustrating the signalthat may be seen at the south terminal of first delay module 855 andfifth module north signal 945 illustrating the signal that may be seenat the north terminal of fifth delay module 875. First module southsignal 915 and fifth module north signal 945 are used by controller 835to interface with the chain of delay modules 855-875 and control thedelay cycle.

Controller 835 begins the delay cycle chain by driving first modulesouth signal 915 below its negative rail for a specified time intervalTsouth. First delay module 855 conducts the integration operation, asshown in the Integrate1 portion of first integrator output signal 920,until Tsouth elapses and first module south signal 915 is released toits negative rail. During the integration operation, first delay module855 stores a charge that is proportional to Vsouth*Tsouth (the voltageVsouth below the negative rail times the predetermined time Tsouthduring which the voltage is driven below the negative rail). Once Tsouthelapses, first delay module 855 performs the deintegration operation, asshown in the Deintegrate1 portion of first integrator output signal 920,for a time period T1 that is proportional to the voltage V1 of firstcell 805. The end of the deintegration operation triggers a markingsignal 950 transmitted through the south terminal of first delay module855, as shown in first module south signal 915. Second delay module 860performs an interleaved or overlapping integration operation, as shownin the Integrate2 portion of second integrator output signal 925, duringtime period T1 while first delay module 855 performs the deintegrationoperation. This process continues in a chain, with each delay moduleexecuting an integration operation during a time period in which theprevious module is executing a deintegration operation, until all delaycycles have been completed and all marking signals have been transmittedthrough the south terminal of first delay module 855. Controller 835 candetermine the relative voltages of the cells based on the time intervalsbetween marking signals in accordance with the following equation, wherecells x and y are any two cells in the delay chain:

Vx*Tx=Vy*Ty

In some embodiments, the north terminal of fifth delay module 875 may beused to provide control signals to the delay modules from controller835. For example, fifth module north signal 945 may be used to controllatching of shunt resistors in delay modules 855-875 to balance thecharge of cells 805-825. In the illustrated exemplary embodiment, fifthmodule north signal 945 is pulled high, as illustrated by signal portion955, during an integration time period of fourth delay module 870. Thisactivates a fourth module shunt signal 960 that causes fourth delaymodule 870 to latch a shunt resistor, which dissipates a portion of thecharge accumulated in cell 820. The latched shunt resistor statepersists until the end of the module's delay period. This may be used tobalance the accumulated charge of cell 820 with the charge of the othercells.

Referring now to FIG. 10, a circuit diagram of an analog delay circuit1000 of a delay module that may be used in the implementation of batterymonitoring system 800 is shown according to an exemplary embodiment.Circuit 1000 may be included as part of one of delay modules 855-875 insome embodiments.

Circuit 1000 includes a positive voltage terminal 1044 and a negativevoltage terminal 1042 across which the battery cell being monitored bycircuit 1000 is connected. Circuit 100 includes a south terminal 1032and a north terminal 1030 through which circuit 1000 communicates withother delay circuits and with a controller. A negative reference voltageis applied to south terminal 1032 for a particular period of time.QSthLo transistor 1006 activates when the voltage at south terminal 1032drops below a ground or negative reference voltage, and integratingsignal 1036 is activated, indicating circuit 1000 is performing anintegration operation. QSouth transistor 1008 activates and theintegrator circuit accumulates charge in capacitor 1020 at a ratedetermined by resistor 1070 and the magnitude of the negative referencevoltage. This time interval is referred to as the integrating timeperiod for circuit 1000. OpAmp 1016 and comparator 1018 are used toperform integration and deintegration in a similar fashion as a dualslope analog-to-digital converter. IntegratorOut signal 1050 is theoutput of the integrator portion of circuit 1000 and has dual slopessimilar to those illustrated in graph 900 corresponding to theintegration and deintegration operations of circuit 1000.

Upon removal of the negative reference voltage at south terminal 1032,circuit 1000 connects its positive voltage rail through QDeInttransistor 1010 to the input of the integrator portion of circuit 1000.This begins removing the accumulated charge within capacitor 1020 at arate determined by the cell voltage and resistor 1070. This timeinterval is referred to as the deintegrating time period for circuit1000. Comparator 1018 detects when all of the accumulated charge hasbeen removed from capacitor 1020, at which point comparator 1018activates QEOD transistor 1024, which in turn activates a −EODPulsesignal 1052, or end of delay marking pulse, as well as a CompareOutsignal 1054. The deintegrating time period divided by the integratingtime period is equal to the magnitude of the negative reference voltagedivided by the cell voltage of the cell being monitored by circuit 1000.With known reference voltage and measured time intervals using markingsignals, a controller can calculate the unknown cell voltage. Becausethe measurements are relative measurements between adjacent cellvoltages, the accuracy of capacitor 1020 and resistor 1070 do not affectthe measurements. The RC values may be selected to produce a particularmeasurement resolution within a specified measurement range andintegration time budget.

Multiple circuits 1000 may be connected together in a chainconfiguration using a single-wire north south daisy chain configuration.In this configuration, the end of an integrating time period for onedelay circuit may trigger the integrating time period for the next delaycircuit in the chain. The negative reference voltage used forintegrating a circuit may be satisfied with the southern neighboringcell delay circuit negative voltage rail. Each circuit makes aconnection to the negative voltage rail for the circuit, which passes tothe northern neighboring circuit during the deintegration time period.This produces a concurrent and equal integration time period for thenorthern neighboring delay circuit.

The chained circuits may output a series of end of deintegration markingpulses whose pulse position intervals are proportional to theirrespective cell voltage and the duration and magnitude of the appliednegative reference voltage applied to the southern most delay circuit.The cell-to-cell comparison of voltages is independent of the appliednegative reference pulse. By using cell-to-cell comparisons, theaccuracy of the fixed reference voltage used to trigger the delayprocess does not affect the voltage measurements, which may reduce thecost of the components needed to generate the negative reference pulse.The calculation of the cell voltages is also not complex.

The end of delay marking pulses generated by each delay circuit may bedigitally propagated to the southern end of the chain for single pointmonitoring and decoding by a controller. Outgoing marking pulsestransmitted through south terminal 1032 may be reflected in DataOutsignal 1034, and incoming data pulses received through north terminal1030 may be reflected in DataIn signal 1058. The propagation delay ofthe signals does not have a cumulative error effect in the end of chaintiming and decoding because the delay circuits share the same digitalpropagation path with the exception of a single delay circuitpropagation delay. Only a single marking pulse is provided for eachdelay circuit, reducing the number of delay pulses and scan time for theentire battery stack without sacrificing measurement resolution by usingconcurrent operations. Because the same cell is providing drive voltagefor both integrating and deintegrating operations at the same time, allvariation and noise experienced by the cell during that period affectsboth operations equally, and the representation of the cell within thecomparative computation performed by the controller is preserved.

The integrating nature of the measurement may allow production of a highresolution measurement in noisy signal environments. The integration ofa signal over time may help filter out high frequency noise. Forexample, noise throughout the battery stack is common in many high powercurrent chopping electric motor drives used for electric vehicles. Acell voltage scan frequency of 100 Hz (10 ms) may be able to absorbnearly all of the cell voltage noise operating at or around 10 to 20 kHzof the current chopper but still be able to update 100 cells connectedin series every second.

Because the measured delays are related to the connection withneighboring cells, in some embodiments, the measurements may be used todetect poor or faulty high current connections. In an embeddedconfiguration, the voltage drop across the high current terminal isinherently included in the voltage measurements without effecting thesupply voltage to the delay circuit. The measurements can be used todetect a poor or faulty connection by comparing the measurements withmeasurements for other cells. During hot plugging of cells or highcurrent terminal connections with a load applied to the stack, highreversal potentials can be present in the delay circuits. These highpotentials can damage the delay circuits if such circuits are notadequately protected. In the embedded configuration, protection for thisunexpected high potential may be placed only on the north south data bussince the delay circuit supply voltage is connected internally withinthe delay circuit, for example via Qblock transistor 1002.

Circuit 1000 also includes a shunt resistor 1014 that may be used todissipate charge from the connected battery cell. A data pulse may bereceived on north terminal 1030 that may activate QNrthHi transistor1028 and, in turn, activate DataIn signal 1058. DataIn signal 1058 maybe used to activate a ShuntOn signal 1040, which activates a QShuntresistor 1012 and latches shunt resistor 1014 across voltage terminals1042 and 1044, dissipating charge from the cell. This may be used tobalance the charge of cells that are at a higher state of charge thanother cells in a battery stack. In some embodiments, the data pulses maypropagate through multiple linked delay circuits and may be injectedduring an active state of a particular circuit to select that circuitfor shunt resistor latching. In some embodiments, incoming data pulsesmay be used for other functions of circuit 1000.

Referring now to FIG. 11, a circuit diagram of a logic portion 1100 ofcircuit 1000 shown in FIG. 10 is shown according to an exemplaryembodiment. Logic portion 1100 illustrates how certain input and outputsignals may be used to trigger other input and output signals in circuit1000.

CompareOut signal 1054 is the output of comparator 1018 and is normallyhigh when inactive. CompareOut signal 1054 goes low when activatedshortly after the integration process begins and stays low until theintegration and deintegration processes end, at which point it becomesinactive and goes high. −Integrate signal 1036 is normally high wheninactive and becomes active (i.e., goes low) when the voltage at southterminal 1032 is pulled below the negative rail, causing the integrationprocess to begin. DataIn signal 1058 is normally low and goes into anactive high state when a northern delay circuit or the controller pullsthe signal at north terminal 1030 above the positive rail voltage,indicating data is being passed into the circuit from a northern delaycircuit. −EODPulse signal 1052 is normally high and goes into an activelow state on the rising edge of CompareOut signal 1054, indicating afull delay cycle has been completed.

−Active signal 1048 is used to indicate when circuit 1000 is in anactive state. −Active signal 1048 is activated when it is at a low leveland is activated when either CompareOut signal 1054 or −Integratingsignal 1036 is active (i.e., when either signal is low). An active−Active signal 1048 indicates that circuit 1000 is performing either theintegration or deintegration operation.

−Delntegrate signal 1038 is normally high when inactive and is activewhen in a low state. −DeIntegrate signal 1038 goes into an active lowwhen −Integrating signal 1036 goes into an inactive high state andCompareOut signal 1054 is still in an active low state, indicating thatintegration has completed but the full delay cycle has not, so circuit1000 is in the deintegration process. IntegrateNorth signal 1056 isinactive when low and active when high and is the inverse of−DeIntegrate signal 1038. IntegrateNorth signal 1056 is used to triggerintegration of the delay circuit to the north of circuit 1000 whilecircuit 1000 performs deintegration. Together, IntegrateNorth signal1056 and −DeIntegrate signal 1038 control the interleaved integrationand deintegration operations of adjacent delay circuits. −DataOut signal1034 is active when low and is activated as a pulse when −EODPulsesignal 1052 is active (i.e., low) or DataIn signal 1058 is active (i.e.,high), indicating that data that is either generated internally bycircuit 1000 or received from north terminal 1030 is being transmittedout of south terminal 1032.

In some embodiments, circuit 1000 may provide a shunt resistor 1014 thatmay be used to dissipate energy from a cell, for example, for chargebalancing purposes. ShuntOn signal 1040 is active when high and isactivated when DataIn signal 1058 is activated, causing shunt resistor1014 to be latched. NOR5 and NOR6 form a set-reset latch that latchesShuntOn signal 1040 into an active state even after DataIn signal 1058goes low. ShuntOn signal 1040 remains in an active state until −EODPulsesignal 1052 goes into an active low state, which resets ShuntOn signal1040 into an inactive low state and unlatches shunt resistor 1014.

Exemplary embodiments presented herein may provide one or more ofvarious advantages. The circuits may utilize simple circuitry and may beimplemented at a relatively low cost per cell. Some embodiments mayprovide a distributive option to reduce electromagnetic interferencepossibilities and the required wire lengths between a management systemand the battery cells. For example, delay circuits may be collocated orembedded within the battery cells as opposed to being remotely orcentrally located. A daisy chain connection may be used betweenneighboring cell circuits, using shorter wire lengths and less overallwiring than a wiring harness connecting the cells to a centralizedcircuit assembly. Embodiments that do not utilize analog-to-digitalconverters (ADCs) have no cell level ADC quantization error because thetime interval is continuous. In some embodiments, the continuous timeinterval may allow for higher resolution conversion at a later point.The circuits may have low parasitic draw on battery cells, no clocks,low data rates, and a naturally passive circuit state. The circuits mayoperate without requiring any digital or sampling synchronization. Insome embodiments, a single wire data bus may be utilized to reduce thenumber of wires that need to be used. Some embodiments may be embeddedor integrated inside a battery stack and/or the battery cells to furtherreduce construction costs and provide hot plugging protection.

The construction and arrangement of the systems and methods as shown inthe various exemplary embodiments are illustrative only. Although only afew embodiments have been described in detail in this disclosure, manymodifications are possible (e.g., variations in sizes, dimensions,structures, shapes and proportions of the various elements, values ofparameters, mounting arrangements, use of materials and components,colors, orientations, etc.). For example, the position of elements maybe reversed or otherwise varied and the nature or number of discreteelements or positions may be altered or varied. Accordingly, all suchmodifications are intended to be included within the scope of thepresent disclosure. The order or sequence of any process or method stepsmay be varied or re-sequenced according to alternative embodiments.Other substitutions, modifications, changes, and omissions may be madein the design, operating conditions and arrangement of the exemplaryembodiments without departing from the scope of the present disclosure.

The present disclosure contemplates methods, systems and programproducts on any machine-readable storage media for accomplishing variousoperations. The embodiments of the present disclosure may be implementedusing existing computer processors, or by a special purpose computerprocessor for an appropriate system, incorporated for this or anotherpurpose, or by a hardwired system. Embodiments within the scope of thepresent disclosure include program products comprising machine-readablestorage media for carrying or having machine-executable instructions ordata structures stored thereon. Such machine-readable storage media canbe any available media that can be accessed by a general purpose orspecial purpose computer or other machine with a processor. By way ofexample, such machine-readable storage media can comprise RAM, ROM,EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic diskstorage or other magnetic storage devices, flash memory, or any othermedium which can be used to carry or store desired program code in theform of machine-executable instructions or data structures and which canbe accessed by a general purpose or special purpose computer or othermachine with a processor. Machine-readable storage media are tangiblestorage media and are non-transitory (i.e., are not merely signals inspace). Combinations of the above are also included within the scope ofmachine-readable storage media. Machine-executable instructions include,for example, instructions and data which cause a general purposecomputer, special purpose computer, or special purpose processingmachines to perform a certain function or group of functions.

Although the figures may show a specific order of method steps, theorder of the steps may differ from what is depicted. Also two or moresteps may be performed concurrently or with partial concurrence. Suchvariation will depend on the software and hardware systems chosen and ondesigner choice. All such variations are within the scope of thedisclosure. Likewise, software implementations could be accomplishedwith standard programming techniques with rule-based logic and otherlogic to accomplish the various connection steps, processing steps,comparison steps, and decision steps.

What is claimed is:
 1. A system for monitoring a battery stack having aplurality of cells, the system comprising: a plurality of time delaycircuits, each of the time delay circuits being electrically coupled toat least one of the plurality of cells, wherein the time delay circuitsare configured to receive a trigger signal, execute a time delay inresponse to receiving the trigger signal, and output a time delaymarking signal indicating that the time delay has elapsed, wherein thetime delay is based on a voltage of the at least one of the plurality ofcells to which the respective time delay circuit is electricallycoupled; and a control circuit configured to receive the time delaymarking signal from each of the time delay circuits and, for eachreceived time delay marking signal, to determine the voltage of the atleast one of the plurality of cells associated with the respective timedelay marking signal.
 2. The system of claim 1, wherein at least one ofthe plurality of time delay circuits is configured to output a referencetime delay marking signal, and wherein the control circuit is configuredto receive the reference time delay marking signal and calibrate thecontrol circuit to determine the voltage for one or more of theplurality of cells based on the reference time delay marking signal. 3.The system of claim 1, wherein a plurality of time delay circuits areconfigured to execute at least a portion of their respective time delaysconcurrently.
 4. The system of claim 3, wherein each of the time delaycircuits is configured to perform an integration operation in whichelectrical charge is accumulated in a capacitor of the time delaycircuit and a deintegration operation in which the capacitor isdischarged, and wherein the system is configured such that one timedelay circuit performs the integration operation while another timedelay circuit performs the deintegration operation.
 5. The system ofclaim 1, wherein the control circuit is configured to transmit a firsttrigger signal to a first time delay circuit, and wherein at least someof the time delay marking signals of the plurality of time delaycircuits are used as trigger signals for some time delay circuits, suchthat the first trigger signal transmitted from the control circuitcauses a plurality of time delay circuits to execute time delays in achain.
 6. The system of claim 1, wherein the control circuit isconfigured to receive the time delay marking signals from the pluralityof time delay circuits via a single input, and wherein outputs of thetime delay circuits are electrically connected to the input of thecontrol circuit in a manner such that an input signal received at theinput of the control circuit indicates the presence of a time delaymarking signal when any of the plurality of time delay circuits outputsa time delay marking signal.
 7. The system of claim 1, wherein thecontrol circuit is configured to determine a comparative voltage levelbetween two cells by using a voltage of one of the cells as a referencevoltage when determining a voltage of the other of the cells.
 8. Thesystem of claim 1, wherein at least one of the plurality of time delaycircuits or the control circuit are configured to be embedded within thebattery stack.
 9. The system of claim 1, wherein the control circuit isconfigured to balance a charge level of the battery cells based on thedetermined voltages.
 10. A method for monitoring a battery stack havinga plurality of cells, the method comprising: for each of a plurality ofsubsets of the plurality of cells, executing a time delay that isproportional to a voltage of the subset of cells, wherein each subset ofthe plurality of cells comprises one or more cells; for each of theplurality of subsets of the plurality of cells, activating a time delaymarking signal when the time delay has elapsed; and for each time delaymarking signal, determining the voltage of the associated subset ofcells based on the time delay marking signal.
 11. The method of claim10, further comprising receiving a reference time delay marking signal,wherein, for each time delay marking signal, the voltage of theassociated subset of cells is determined based on the time delay markingsignal and calibrated using the reference time delay marking signal. 12.The method of claim 10, wherein the time delays for the plurality ofsubsets of cells are executed in a chain and the time delay markingsignal of one subset of cells is used to trigger execution of the timedelay for the next subset of cells.
 13. The method of claim 10, furthercomprising determining a comparative voltage level between two cells byusing a voltage of one of the cells as a reference voltage whendetermining a voltage of the other of the cells.
 14. The method of claim10, further comprising embedding circuitry within the battery stack thatis configured to execute the time delays, activate the time delaymarking signals, and determine the voltages.
 15. The method of claim 10,further comprising balancing a charge level of the battery cells basedon the determined voltages.
 16. A system for monitoring a battery stackhaving a plurality of cells, the system comprising: means for executing,for each of a plurality of subsets of the plurality of cells, a timedelay that is proportional to a voltage of the subset of cells, whereineach subset of the plurality of cells comprises one or more cells; meansfor activating, for each of the plurality of subsets of the plurality ofcells, a time delay marking signal when the time delay has elapsed; andmeans for determining, for each time delay marking signal, the voltageof the associated subset of cells based on the time delay markingsignal.
 17. The system of claim 16, further comprising means forreceiving a reference time delay marking signal, wherein, for each timedelay marking signal, the voltage of the associated subset of cells isdetermined based on the time delay marking signal and calibrated usingthe reference time delay marking signal.
 18. The system of claim 16,wherein the means for executing the time delays for the plurality ofsubsets of cells is configured to execute the time delays in a chain andthe time delay marking signal of one subset of cells is used to triggerexecution of the time delay for the next subset of cells.
 19. The systemof claim 16, further comprising means for determining a comparativevoltage level between two cells by using a voltage of one of the cellsas a reference voltage when determining a voltage of the other of thecells.
 20. The system of claim 16, wherein the means for executing,means for activating, and means for determining are embedded within thebattery stack.
 21. The system of claim 16, further comprising means forbalancing a charge level of the battery cells based on the determinedvoltages.
 22. The system of claim 16, wherein the means for determining,for each time delay marking signal, the voltage of the associated subsetof cells is configured to receive the time delay marking signals via asingle input, wherein the single input is configured such that an inputsignal received at the input of the control circuit indicates thepresence of a time delay marking signal when any of the time delaymarking signals are activated.